drm/ast: Remove gamma LUT updates from DPMS code

The DPMS code, called from the CRTC's atomic_enable, rewrites the
gamma LUT. This is already done by the CRTC's atomic_flush. Remove
the duplication.

v2:
- fix a typo in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-6-tzimmermann@suse.de